1. Field of the Invention
This invention relates to a semiconductor memory device having memory cells including ferromagnetic films and a control method thereof. For example, this invention relates to a read operation of a magnetic random access memory (MRAM: Magneto resistive Random Access Memory).
2. Description of the Related Art
An MRAM is a device which performs a memory operation by selectively storing “1” or “0” information by use of the magneto-resistive effect. The MRAM has a nonvolatile property, high integration density, high reliability and high operation speed and is seen as a memory device which can be used instead of a conventional DRAM (Dynamic Random Access Memory), EEPROM (Electrically Erasable and Programmable Read Only Memory) or the like.
A memory cell in an MRAM has a structure which is formed by laminating a plurality of ferromagnetic films. Information is stored by setting binary information items to correspond to parallel or anti-parallel relative directions of magnetization of a plurality of ferromagnetic films configuring the memory cell. The write operation of the MRAM is performed by reversing the magnetization direction of the ferromagnetic films of each cell by use of a current magnetic field caused by passing a current through write lines arranged in a cross-stripe form. The MRAM is a nonvolatile memory in which the power consumption at the stored information holding state is zero in principle and the stored information holding operation is performed even if the power supply is turned OFF. The read operation of recorded information is performed by use of a phenomenon that the electrical resistance of the memory cell varies depending on a relative angle between the magnetization direction of the ferromagnetic film of the cell and the sense current or a relative angle between the magnetization directions of a plurality of ferromagnetic layers. The phenomenon is called the magneto-resistive effect.
For example, an MRAM has the following advantages if the function thereof is compared with that of the conventional semiconductor memory using a dielectric film.
(1) It is completely nonvolatile and 1015 or more rewrite operations can be attained.
(2) A nondestructive read operation can be performed and the restore operation is not required. Therefore, it is possible to shorten the readout cycle.
(3) It is highly resistant to radioactive radiation in comparison with a charge storage memory cell.
It is expected that the integration density per unit area of the MRAM and the write/read time thereof are approximately equal to those of the DRAM. Therefore, it is expected to apply the MRAM to external memory devices of portable instruments, use the same in a embedded form on an LSI or apply the same to main memories of personal computers by utilizing the nonvolatile property thereof.
In the MRAM which is being now studied for practical use, an element which exhibits a tunnel magneto-resistive effect (which is hereinafter referred to as a TMR effect) is used for the memory cell and it is described in “IEEE International Solid-State Circuits Conference 2000 Digest of Technical Papers” TA7.2, for example. The element which exhibits the TMR effect is configured by a three-layered film mainly including a ferromagnetic layer/insulating layer/ferromagnetic layer and a current tunnels and passes through the insulating layer. The tunnel resistance varies in proportion to the cosine of a relative angle between the magnetization directions of the two ferromagnetic metal layers and becomes maximum when the magnetization directions are anti-parallel. For example, in the case of the NiFe/Co/Al2O3/Co/NiFe tunneling junction (MTJ: Magneto Tunneling Junction), the magneto-resistive change rate of 25% or more in the low magnetic field of 50 Oe or less is detected. This is described in “IEEE Transactions on Magnetics”, 1997, No. 33, p. 3553, for example.
Further, various proposals have been made so as to improve the characteristics of the MRAM using the MTJ element. For example, an MTJ structure in which an anti-ferromagnetic layer is arranged adjacent to one ferromagnetic layer and the magnetization direction is fixed so as to improve the magnetic field sensitivity is proposed in “Japanese Journal of Applied Physics” 1997, No. 36, p. 200. The MTJ structure is called a spin valve structure. Further, an MTJ structure having double tunnel barriers so as to improve the bias-dependency of the magneto-resistive change rate is proposed in “Japanese Journal of Applied Physics” 1997, No. 36, p. 1380. Further, a structure having a keeper layer or yoke structure which is formed of a magnetic material with high permeability around the wirings is proposed in U.S. Pat. Nos. 5,940,319, 5,956,267, European Patent Specification No. 00/10172 and Jpn. Pat. Appln. KOKAI Publication No. 8-306014. The structure is made to concentrate the magnetic field in a portion near the MTJ and reduce a write current by converging the magnetic flux in the keeper layer or yoke structure. Also, a structure in which the arrangement of wirings and MTJ are changed so as to reduce the write current is disclosed in U.S. Pat. Nos. 5,946,228, 6,072,718, 6,104,633 and 6,005,800. Further, the fact that the write threshold value of the magneto-resistive element has temperature dependency is described in “IEEE Transactions on Magnetics” 2001, No. 37, p. 1970 and “2002 Symposium on VLSI Circuit Digest of Technical Papers”, 12–3.
As described above, conventionally, the MRAM is being actively studied as a next-generation memory device. However, the conventional MRAM has problems caused by interference between adjacent cells when the data is written into the memory cell.
In the conventional MRAM, a plurality of write lines are arranged to substantially intersect with one another and each MTJ element is arranged at an intersection of two write lines. Currents are supplied to two write lines connected to a selected memory cell at the write time. As a result, data is written into the selected memory cell by an influence of the resultant magnetic field due to the currents flowing in the two write lines. At this time, data may be written into a non-selected memory cell which is adjacent to the selected memory cell by an influence of the magnetic field applied to the selected memory cell in some cases.